1. Field of the Invention
This invention relates to vacuum heat treating furnaces and in particular to a vertically-oriented vacuum furnace and an apparatus for lifting a work load into the vacuum furnace and lowering therefrom.
2. Description of the Related Art
Industrial vacuum heat treating furnaces having either a horizontal configuration or a vertical configuration are known. In a vacuum furnace having a horizontal configuration, a work load of parts to be heat treated is transported into the furnace chamber with an apparatus that provides horizontal translation of the work load. In a vacuum furnace having a vertical configuration, a lifting apparatus is used to raise the work load from the factory floor up to the furnace chamber which is elevated.
A known arrangement for a lifting system for a vertical vacuum furnace utilizes a four-point lifting apparatus. The apparatus typically includes four ball screws that operated synchronously so that the work load is lifted evenly. In order to keep the ball screws in synchronism with each other, multiple gear boxes and connecting shafts with couplings are utilized. The known designs for such lifts were composed of many parts that had to be assembled and aligned at the manufacturing site, and then disassembled for shipment. When the furnace arrives at the customer site, the lift apparatus must be re-assembled and aligned again. That is a time consuming process that usually adds several days to the delivery time schedule.
The gear boxes, drive shafts, and couplings used in the known lift mechanisms generate a considerable amount of noise when operating to lift or lower a work load. The couplings that connect the drive shafts, motor shafts, gear box shafts, and the ball screw shafts become loosened over time. When that occurs, it causes one or more of the ball screws to become un-synchronized with the other ball screw(s). Such out-of-synch operation can cause catastrophic damage to the lifting mechanism. If the ball screws get too far out-of-synch, the work load itself and even the hot zone inside the furnace can be damaged.
The known lift mechanisms for vertical vacuum furnaces have lifting points that contact the bottom lifting structure of the furnace through coil springs. The lift mechanism is operated to lift the bottom door toward the furnace until a mechanical limit switch is tripped, thereby providing an indication that the lifting structure was in its final, fully-lifted position. In the final lifted position, the springs are compressed a small amount as the bottom lifting structure contacts the upper part of the furnace vessel. If the mechanical switch is not adjusted properly or becomes out-of-adjustment, the springs over-compress and the lifting structure and door can be subject to bending damage.
In view of the foregoing problems with the known lifting systems for vertical vacuum furnaces it would be desirable to have a lifting apparatus for a vertical vacuum furnace that overcomes the problems associated with the known lifting systems.